Process of refining mineral oil



July 27, 1937.

Fig.

Fig.2.

F. M. CLARK 2,088,405 PROCESS OE'REFINING MINERAL OIL original Filed May l9, 1934 r A320 Was/yr T/ME IN DflYS Inventor:

Frank M. Clark,

His Attorney.

Patented July 27 1937 rnoonss 0F REFINING MINERAL OIL Frank M. Clark, Pittsfield, Mass, assignor to General Electric Company, a corporation of New York Original application May 19, 1934, Serial No. 726,498. Divided and this application February 12, 1935, Serial No. 6,

6 Claims.

The present invention comprises an improved method of refining mineral oil compositions with the main object in view of producing an oil which is particularly well suited for insulating and dielectric purposes. This application is a division of my prior application Serial No. 726,498, filed May 19, 1934.

Crude mineral oils and especially those of the naphthemc type which are obtained from the Gulf coast and mid-continental petroleum fields contain a high percentage of both unsaturated aliphatic and unsaturated aromatic hydrocarbons. The formation in mineral oil during use of insoluble gummy and resinous masses (commonly called sludge), and the instability of mineral oil when subjected to heat and exposed to air and light is to be ascribed to the presence in the oil of these unsaturated compounds. It has been the object of refining processes to remove unsaturated compounds as completely as possible.

Heretoiore the general practice in the refining of mineral oils has involved initial treatment at high temperatures with concentrated (or fuming) sulphuric acid or else with phosphoric acid. It has also been proposed to associate the sulphuric acid with boric acid.

Although these prior methods are of value in the treatment of certain types of oils, they all possess certain disadvantages. Mineral oils treated in accordance with the previously proposed methods are apt to be either under-refined or over-refined and hence not well suited for use in an insulating or dielectric capacity. For example, under-refined oils contain unsaturated aliphatic hydrocarbons, such as olefin compounds, which by oxidation and polymerization form sludge. When under-refined oil is used in transformers, any sludge which may be formed obstructs the circulation of the oil and reduces the desired cooling effect. A transformer in this condition is easily overheated and caused to fail. Over-refined oils on the other hand contain. substantially no unsaturated compounds. Oils of this type, although not subject to sludge formation, are easily oxidized on exposure to air and soluble products which are highly acidic and corrosive in nature are formed. When such oil is used in a transformer the acid products form permanent emulsions with the oil and water which may be present. Water may be formed by oxida-, tion or may enter the transformer during. operation. Such emulsions hasten electrical breakdown. Under-refined oils and over-refined oils because of the formation of sludge and acid products respectively likewise have been found unsuited for other electrical application, for example, as cooling or dielectric media in cables and capacitors.

When crude oils are refined by treatment with concentrated sulphuric acid alone both aliphatic and aromatic unsaturated compounds are removed. As the sulphuric acid combines with the aliphatic and aromatic unsaturated bodies equal- 1y well, it isvimpossible to remove one type of unsaturated compounds without also removing the other. Transformer oil in use today refined by the sulphuric acid process is a compromise oil. It contains both the sludge-forming and acidproducing compounds. Oils treated with phosphoric acid are subject to practically the same disadvantage. When the sulphuric acid is associated with boric acid the unsaturated aromatic component of the oil is less attacked but likewise the effect of the sulphuric acid on the aliphatic unsaturated components is reduced. As is well known, transformer and other oils used in an insulating and dielectric capacity commonly have a Saybolt universal viscosity at F. not over about 200 seconds, for example, between about 60 and seconds.

In accordance with my present invention I have provided a method of producing a mineral oil which is particularly Well suited for electrical insulating and dielectric purposes. The product resulting from the herein described refining treatment contains less, than one per cent by volume of olefinic (aliphatic) hydrocarbons and about 4 to 8 per cent by. volume of residual unsaturated aromatic hydrocarbons.

In accordance with my invention I have provided a process for purifying and refining crude mineral oilsto produce my improved oil product by the treatment of the crude oil with an acid mixture composed preponderantly of sulphuric acid andcontaining a minor proportion of phosphoric'acid. An'acid mixture containing the proper amounts of sulphuric acid and phosphoric acid attacks the olefinic constituents preferentially, resulting in a product which contains the minimum of olefinic unsaturation and the desired amount of unsaturated aromatic compounds. I

The oil resulting from this treatment is stable, and is particularly adapted for insulating and dielectric purposes. It contains less than one per cent by volume of olefinic (aliphatic) hydrocarbons and about 4 to-8 per cent by volume of unsaturated aromatichydrocarbons.

In Fig. 1 of the accompanying drawing, graphs are represented showing the relation of sludge formation when crude mineral oils were treated with varying amounts of diiferent refining agents. In Fig. 2 are represented graphs showing the rate of sludge accumulation in crude oil and in oil refined by various methods.

When in accordance with my invention the oil is treated with a mixture of sulphuric acid and phosphoric acid, the procedure preferred is as follows: 5 pounds of an acid mixture composed of approximately per cent commercially concentrated sulphuric acid (for example, sp. gr. 1.84, 96% strength) and approximately 15 per cent crthophosphoric acid (for example, sp. gr. 1.71, 85% strength) is added slowly with continuous agitation to 55 gallons of crude mineral oil. The rate of addition is such that the temperature will not rise higher than about 35 C. Preferably the temperature of the oil-acid mixture should be kept below 25 C. The oil-acid mixture is allowed to stand until the resulting sludge is settled out. The supernatant refined oil is separated, Washed with water or with a weak alkaline solution in order to neutralize any free acids left in the oil. A 3 per cent sodium hydroxide solution is preferred for the alkaline wash. Weaker or stronger solutions may be used although the alkaline solution should not be higher than 5 per cent in concentration. The refined oil is dehydrated and then may be subjected to a fullers earth treatment, although the latter treatment is not indispensable.

The oil treated by a properly balanced refining acid mixture as above described is extremely stable and is neither under-refined nor over-refined. An oil which is especially suited for insulating application shows an aliphatic (olefinic) unsaturation of less than 1 per cent and a much larger amount of unsaturated aromatic compounds ranging from 4 to 8 per cent.

The unsaturated aromatic compounds stabilize the oil against oxidation and sludge formation, reduce to a minimum the soluble acidic products of oxidation, and stabilize the oil against ionization effects when subjected to electric stress which is capable of causing a splitting oil of hydrogen and the polymerization of the hydrocarbon residue with the formation of insoluble waxy materials.

In Figure 1 of the drawing, graph A represents characteristics of oil treated with sulphuric acid, graph B represents characteristics of oil treated with the sulphuric acid-boric acid mixture and graph C represents characteristics of oil treated with the sulphuric acid-phosphoric acid mixture in accordance with my invention. The percentage of sludge by weight accumulated on the 35th day was determined and plotted as ordinates, the amount of refining agent being plotted as abscissae. As shown by graph A increased amounts of sulphuric acid produced oil having correspondingly lower sludging characteristics. The best results for the sulphuric acid-boric acid mixture (graph B) were obtained when 10 lbs. of the acid mixture (containing 2.7% boric acid) were used with 55 gallons of crude oil. As shown by the graph C oil refined by treatment with the sulphuric acid-phosphoricacid mixture (containing 15% phosphoric acid) resulted in an oil having lower sludging characteristics than oil refined by either of the other two methods. The best results as shown by this graph were obtained using 5 lbs. of the sulphuric phosphoric acids mixture per 55 gallons of crude oil but, as shown in Fig. 1, effective results are alsolobtained when the oil is treated with the sulphuric-phosphoric acidsmixture in the ratio of about 4 to 15 lbs. of said mixture per 55 gallons of crude oil.

Fig. 2 represents by difierent graphs the percentage of sludge by weight formed during a given time with different refining agents using an optimum amount as indicated in Fig. 1. ,Graph D represents the sludging characteristics of crude oil, graph E represents that of oil purified by sulphuric acid (15 lbs. sulphuric acid per 55 gallons of oil), graph F represents that of oil purified by a sulphuric acid-boric acid mixture (10 lbs. acid mixture containing 2.7% boric acid per 55 gallons of oil), and graph G represents the sludging characteristics of oil purified by a sulphuric acidphosphoric acid mixture (5 lbs. acid mixture containing 15% phosphoric acid per 55 gallons of oil). Crude oil refined by the sulphuric acidphosphoric acid mixture exhibits superior nonsludging characteristics although smaller amounts of the refining agent were employed.

The following method was used to determine the sludging characteristics of the various oils represented in the graphsof Figs. 1 and 2. 55 cc. of the'oil were placed in a Pyrex test tube containing a copper spiral and heated in an oven at C. The test tubes were removed at definite intervals, the oxidized oil was diluted with an equal amount of naphtha and then was centrifuged. The sludge obtained was washed by repeated naphtha additions and centrifuged until free from oil, after which the weight of the sludge was determined gravimetrically and expressed in per cent by weight of the original oil.

By the term crude mineral oil as used herein I mean a non-refined or a semi-refined oil stock which, when treated as herein described, is adapted for use in an electrical insulating or dielectric capacity.

Although I have described my invention setting forth the purification and refinement of crude mineral oils in general using specific proportions of my acid mixture, it will be understood by those skilled in the art that some types of crude. oils may require modifications in the acid ratio. All modifications of my process coming within the true spirit and scope of my invention are meant to be covered in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A method of refining crude mineral oil which comprises treating the oil at a temperature not substantially exceeding 35 C. with a mixture comprising approximately 85 per cent concentrated sulphuric acid and approximately 15 per cent concentrated phosphoric acid, and removing acidic compounds and uncombined acid.

2. A method of treating crude mineral oil which comprises agitating the oil at a temperature not substantially exceeding 35 C. with a reagent consisting of about 85. per cent concentrated sulphuric acid and about 15 per cent concentrated phosphoric acid.

'3. The method of treating crude mineral oil preparatory to its use for electrical insulating and dielectric purposes which consists in bringing such oil into contact with a mixture of approximately 85 per cent'concentrated sulphuric acid and approximately 15 percent concentrated orthophosphoric acid at a temperature below about '35" C., allowing the-mixture to stand until the resulting sludge settles, removing the supernatant oil and removing acid'and water therefrom.

4. A process of treating crude mineral oil which comprises agitating said oil with a reagent consisting of about 85 per cent concentrated sulphuric acid and about 15 per cent phosphoric acid of about 85 per cent acid strength, said reagent being applied to said oil in the ratio of about 4 to 15 pounds of reagent to about 55 gallons of oil, and suppressing substantial rise in temperature.

5. A process of treating crude mineral oil which comprises agitating said oil with a reagent consisting of about 85 per cent concentrated sulphuric acid and about 15 per cent phosphoric acid of about 85 per cent acid strength, said reagent being applied to said oil in the ratio of about 5 pounds of reagent to about 55 gallons of oil, and. preventing the temperature from rising above about 35 C.

6. The step in a process of refining crude mineral oil for electrical application which comprises treating said oil with an acid mixture consisting of about 85 per cent concentrated sulphuric acid and about 15 per cent orthophosphoric acid of about 85 per cent acid strength, said acid mixture being applied to said oil in the ratio of about 5 pounds of acid mixture to about 55 gallons of oil.

FRANK M. CLARK. 

